However, if you can find a spice model for your target device, then you can edit the relevant parameters of one of the built-in CL models and use that in your circuit.

SPICE parameters for devices have little to do with datasheet specifications. They are much more to do with semiconductor physics and the particular process under which a given device is manufactured at the foundry.
Consequently it is not always a simple task to map datasheet information onto the model parameters.

If you want to try to understand more about what the SPICE - and hence CL - parameters mean in diodes, bipolar transistors and MOSFETs, then you might like to have a look at:

Clarification: "Is there a listing somewhere describing or defining the MOSFET parameters that can be changed." My question was specifically related to CircuitLab. I know about MOSFET and about the various parameters of MOSFETs.

The generalized model is preferable over that of a specific MOSFET.

Within CL, one can pop-up the MOSFET parameter window which displays the parameters that can be changed within CL. Of these, I am/was asking about clarification of the following three that are on the MOSFET parameter change window within CL:

Each capacitance is measured at with the device biased at some specified Vgs (gate to source voltage) and Vds (drain to source voltage) but with either:

i) the other two terminals shorted at AC by connecting a large very low inductance and low effective series resistance capacitance across those two pins;

or;

ii) one of the terminals connected directly or decoupled to an AC ground by connecting a large very low inductance and low effective series resistance capacitance from the pin to the AC ground.

C_oss is drain to gate capacitance with gate shorted to source at AC.
C_iss is gate to drain capacitance with drain shorted to the source at AC.
C_rss is drain to gate capacitance (with source connected to ground).

K_P seems as "transconductance" but its unit is A/V^2. The unit of transconductance is A/V. If you build a simple NMOS circuit such as a diode connected one with an ideal current source where Vgs=Vds, you never get the correct value of Vgs for a fixed current by using gm=2Id/(Vgs-Vth).

If you click on the link I sent simulate that circuit for DC you will see that Id=1/2MobilityCoxW/L(Vgs-Vth)ˆ2 is satisfied if K_P is defined as MobilityCoxW/L by CircuitLab different from Spice..

I wonder what your feedback is after you simulate the circuit. Just change Id and measure Vds=Vgs you will see that the numbers make sense only when K_P parameter is thought as MobilityCoxW/L rather than the conventional Spice way.

It is also questionable why there is no W and L as design parameters. It should be included..

I'm struggling to get a grip on K_P. If I drag & drop an N-Channel MOSFET The default value is 5. Using the default parameters, the simulation is reasonably close to the actual performance of the circuit on the bench. However, the value of K_P from the Spice model for the actual MOSFET I am using is 1.04E-6, but if I enter that in the parameter box I get absurd results. In fact if I enter 11, which is the typ Gfs from the datasheet I get even closer results. I understand that Gfs from the datasheet is in A/V whereas K_P is A/V[squared]. Can anyone explain this? Thanks.

I don't know why, but the URL on the Ixys web site does not change as I drill down to the actual Spice model file. Anyway, if you feel inspired, the basic URL is:
http://www.ixys.com/TechnicalSupport/pspice.aspx
Then select DISCRETE MOSFETs and N-Channel Depletion-Mode MOSFETs. You can then download the PSPICE file. Thanks.